The tin whiskers may actually be the modality of failure at work here, Bobalu. It would explain all of the symptoms. Whiskers growing from both directions across such small gaps will, occasionally touch under both minor flexing and also under heat expansion resulting in intermittent shorting of the IC connections. It would explain the flashing, irregular grey bar. Arcing, even at micro voltages and amperages,acts like that as it connects and breaks the connection.
Yup, those were my thoughts as well.
I’d definitely reball with lead bearing solder.
Damned EU solder-free regulations have left us with this mess.
Just need a good dose of EMP to burn those micro-whiskers away! ;-)